Add maintenance status to Dashboard

[blfgomes]

I don't know if it was a design decision not to include the maintenance status in the dashboard (or the 'Status of triggers' screen resource, since they are the same), but I think it belongs there as well.

Here's my patch to enable it (the file being modified is include/blocks.inc.php).

[Aly]

Patch applied with changes rev.10872. Thank you.